Polyimides find extensive use in electronic applications where they are useful in forming dielectric films on semiconductor devices. They are particularly useful as protective coatings for semiconductor devices.
It is well known in the polymer art to make thermally stable all-aromatic polyimides by the condensation polymerization of dianhydrides and diamines to form polyamic acid. Such polyimide precursors are disclosed inter alia in U.S. Pat. No. 3,179,634 to Edwards. These polyamic acids are readily dehydrated to the corresponding polyimides by heating at high temperatures, e.g. 300.degree. to 400.degree. C. These all-aromatic polyimides are not completely soluble and, therefore, protective films cannot be coated directly in the polyimide form. Instead, it is customary practice to use instead the precursor polyamic acids which have the advantage of being readily soluble in aprotic solvents. Such polyamic acid solutions are then coated to form the appropriate film and converted to the corresponding polyimide by heating the film at high temperature to effect volatilization of the solvent and dehydration of the polyamic acid to form the polyimide.
In the use of polyimide coatings as insulating layers over electronic devices, it is extremely important that the layer be as nearly planar as possible so that uniform overlying functional layers can be readily applied. As used herein the term "planarity" refers to the ability of the material to flow over structures of various shapes and produce a planar surface. In other words, the polyimide layer should be flat and not exhibit the surface imperfections of the underlying substrate and/or electronic components. This is extremely difficult to do for the reason that, as solvent is evaporated during the conversion of the polyamic acid to the polyimide form, the polymer shrinks and rapidly tends to become rigid and will not readily flow out to form a planar layer. In copending U.S. patent application Ser. No. 650,913 filed concurrently herewith. Fryd and Merriman disclose a method for making such planar coatings. While these coatings are very effective, their utility may in some instances be restricted because they are susceptible to solvent attack. However, the usefulness of such coatings would be enhanced still further if they could be made solvent resistant.